$3.$ At time t$,$ the vector r $=4$x $^2*$ i $-(2$x $+6$t $^2)*$ j gives the position of a $5\mathrm{.}0$ kg particle relative to the origin of an sy coordinate system $($is in meters and r is in seconds$).($a$)$ Find the magnitade of the particle's angular momentum relative to the origin at time t $=4$ secs$.($b$)($a$)$ Find the magnitude of the particle's torque relative to the origin at time t $=4$ secs$.$